Lipids are substances that are insoluble or poorly soluble in water. This means that it cannot dissolve in water. Lipids as a group comprises fats, oils and waxes. Apart from being produced in the body, lipids are an essential component of the human diet as it is needed for a number of purposes to maintain health (homeostasis).
Types of Lipids
Fatty acids are the main building blocks of the various groups of lipids. In the human body, the main lipids that are of importance are triglycerides, phospholipids and cholesterol. Triglycerides and phospholipids are made up of fatty acids and glycerols and are commonly referred to as fats. Cholesterol is a waxy sterol and is made up of parts of fatty acids like acetyl coenzyme A (acetyl-CoA).
Transport of Lipids
Lipids cannot be transported in the blood in its natural form because it is insoluble or poorly soluble in water. By bonding to lipoproteins, it can travel through the blood stream to various tissues or to the liver. These lipoproteins are primarily produced by the liver and are composed of lipids and proteins. Lipoproteins are classified according to their density :
- VLDL – very low density lipoprotein
- IDL – intermediate density lipoprotein
- LDL – low density lipoprotein
- HDL – high density lipoprotein
Another type of lipoprotein is a chylomicron which mainly transports lipids absorbed from the gut to adipose tissue (fat tissue) and the liver via the blood stream. Lipoproteins carry triglycerides, phospholipids and cholesterol with VLDL’s carrying the most triglycerides and LDL’s carrying the least. LDL’s which are high in cholesterol play an important role in the development of certain diseases like atherosclerosis and heart disease. Therefore LDL is commonly referred as a “bad” cholesterol while HDL is known as the “good” cholesterol because it carries cholesterol to the liver to be metabolized and passed out in bile.
Lipids in Foods
Triglycerides that are solid at room temperature are known as fats while those that are liquid are referred to as oils. Fats are primarily found in animals while oils are found in both plants and fish. Due to changes in the carbon bonds, fatty acids may be saturated or unsaturated. In the natural form, saturated fats have a higher melting point and therefore exist more often as fats while unsaturated fats have a lower melting point and more frequently occur as oils. However, due to modern processing methods for food production, some oils like vegetable oil are saturated fats.
With unsaturated fatty acids, changes in the number of hydrogen atoms bonded to it make it monounsaturated or polyunsaturated fatty acids. Generally, saturated fats are considered to be “bad” fats as it raises the LDL levels in the blood while unsaturated fats reduce cholesterol levels and are known as “good” fats. However, a type of unsaturated fat known as trans fats are considered “bad” since it raises LDL levels and lowers HDL levels.Some fatty acids are known as essential fatty acids because it is necessary for the body to function properly.
Function of Lipids
Lipids play many important roles in the body. Apart from being produced in the body, it is also acquired through the ingestion of food. It is a major source of nutrition and provides more energy ounce for ounce than other foods like carbohydrates and protein.
The main functions of lipids in the body are to provide energy and form the membranes of cells and organelles. These functions consume most of the lipids in the body. Other equally important functions that consume smaller amounts of lipids includes the formation of bile salts, production of hormones, and creating a waterproof and insulating barrier in the skin. Lipids also play an important role in cushioning the body and vital organs by absorbing force (shock).